With the rapid advances being made in capacity and throughput of electronic systems, the demand for display resolution continues to increase. This will likely continue until display systems are capable of approaching the visual acuity and perceptual limits of the human eye. At the same time, such systems must be practical, having acceptable cost, size and efficiency characteristics.
One of the technologies currently receiving significant attention is the active matrix liquid crystal display (AMLCD). The popularity of liquid crystal (LC) technology stems from a variety of factors, most notably the flexibility of the technology to support a variety of sizes, configurations, costs and markets. LCDs span the range from wristwatch displays to laptop computer displays. High-end AMLCDs are being used in demanding avionics applications, and compact, high-resolution light valves are being developed for head-mounted and large screen projection displays. Application areas can be further identified by whether large direct view systems are indicated or whether small, compact projection sources are the best option.
A rather large number of LCD operating modes have been developed, indicative of the flexibility inherent in this technology. Currently, LCDs of different types compete against each other as well as against alternate display methods such as cathode ray tubes (CRTs).
Of the various LCD methods, the AMLCD is generally considered to provide the best overall display performance. By incorporating an active non-linear element, such as a thin film transistor (TFT) latch circuit at each pixel, the multiplexibility of the display is increased more readily than with passive methods. This performance comes, however, at the penalty of complexity and cost in producing the display panels. Hence, the passive devices have generally lower performance at lower cost, while the active matrix LCDs have the highest image quality but at higher cost. The challenge for both types, as well as other display technologies, is to meet existing market demands for quality and cost, as well as to push the limits on quality to open up new applications. The AMLCD is well poised to extend the resolution and performance barriers to satisfy future needs, especially in compact and medium sized displays.
Two parameters with particular relevance to enhanced LCD performance are the resolution capability and optical efficiency. Cost must also be competitive, especially for applications where there are viable alternate methods. Due to the processing complexity, AMLCD aperture ratio is typically given up to allow for improved performance and yields. Aperture ratio is the fraction of the total pixel area which actually modulates light. For small projection displays, aperture ratio is even more subject to tradeoffs due to the limited amount of total pixel area. As a result, typical aperture ratios range from 60 percent or so in large displays down to around 30 percent or less at resolutions up around 1000 lines per inch (lpi).
Future requirements can be anticipated to approach 75 pixels per degree or more at normal viewing distance. Providing this over a 50 degree field of view requires pixel counts in the vicinity of 4000 color pixels on a side. This raises interesting challenges, especially in the case of compact light valves. Aperture considerations loom as a considerable obstacle.
The conventional approach to increasing pixel density is to scale the pixel down. Unfortunately, not everything scales as desired. In the addressing matrix itself, examples include parasitic capacitances and contacts. In the liquid crystal portion of the device, the impact of LC alignment anomalies does not always scale with the pixel pitch. These artifacts are caused in part by electric fields around the active matrix addressing structures. Often, these alignment anomalies are covered up by a black matrix masking layer to prevent them from adversely affecting the contrast of the display. As the pixels are made smaller, the footprint of such structures as the black matrix take up a proportionately larger fraction of the available space, and the useful aperture can be decreased considerably.
Other LCD operating modes have been implemented which circumvent these aperture limits to varying degrees. Passive matrix approaches using transparent addressing lines can have high transmittance, but performance is otherwise limited in a variety of ways. Reflective structures have been used, but the reflection mode projection systems are generally more restrictive than transmissive systems, and the LCD operating modes used have been less desirable than the conventional transmissive twisted nematic and its variations.
For monochrome and additive color systems, the loss of aperture reduces the efficiency proportionately, and eventually makes higher resolutions impractical. The impact is even more pronounced in stacked, subtractive color systems, where the transmittance of the stacked image source can vary as the cube of the aperture ratio. Other optical effects associated with the presence of the stacked matrix structures are also strongly dependent upon the aperture ratio.